The U.S. Navy Diving Manual authorizes divers to breathe oxygen for up to 240 minutes per day at depths of 20 feet of seawater (fsw) or less but does not address the possible accumulation of effects over multiple days. When we conducted experimental four-hour dives with oxygen partial pressure (Po2) of approximately 1.4 atmospheres (atm)2,3 with divers at rest, we concluded that the limit of 240 minutes in 24 hours is acceptable for multiple days. However, when divers exercised for 120 minutes of their underwater time, mild pulmonary effects accumulated.4 The question addressed in this component of the NAVSEA-funded task, Pulmonary Oxygen Toxicity While Swimming: How Exercising Underwater or Using a Rebreather Underwater Breathing Apparatus Affects Pulmonary Function After 1.35 Atm Oxygen Exposures,5 was whether pulmonary effects accumulated over five days of three-hour dives when half of the dive time was spent exercising.